A blog for various doodles, tips, and tricks of embedded Linux development.

23 Feb 2015

Bash Prompt Command Duration

My current bash prompt is fairly useful, it tells me on which machine it's running (useful for situations where I'm ssh'ed out to different places or running in one of multiple text-based VMs), it tells me which under which user I'm currently running, and it indicates where I am in the directory structure. If I happen to be located in a git repository it'll also tell me which branch is currently checked-out, and whether or not my working directory is clean.

A couple years back I added a timestamp: every time bash prints out my prompt, it prints out the date and time it did so. I found that I would write code using one or more terminals and I'd have another terminal for "make check". Since the testing could take a while I would often go off and do other things while waiting for it to complete. Sometimes I would go off for so long that, upon returning, I couldn't remember whether or not the tests were run after my most recent code changes! Comparing the time stamps on the various terminals (or on the code files) would quickly remind me whether or not the tests were run.

In recent years I've also taken to using "pushd" and "popd" quite a bit. In a strange way, pushing into other directories has sort of become a rather crude "todo list" in my workflow. I'll be working on task A, but then realize something else needs to be done. I'll "pushd" somewhere else to do task B and, if I remember to "popd", be brought back to task A. Trying to remember how many levels deep I was became difficult, so I added a "dirs:" count to my bash command line to keep track of how many times I had "pushd".

Since the pushd count proved so useful, I also added something similar to keep track of how many background tasks a given terminal had spawned; a "jobs:" count.

Recently I've come to realize that there's another piece of information I'd like my bash prompt to keep track of for me: what was the duration of the last command I just ran?

Often I'll do a "du -sh <dir>" on some directory to see where all my disk space is being eaten up. Often I can guess which directories contain few bytes and which contain a lot. But sometimes an innocuous looking directory will surprise me and "du" will take a long time. When it does finish my first question is invariably: how long did _that_ take?! Also I tend to do a lot of builds, and I'm always curious to know how long a build took. For commands that produce little output, figuring out the duration is easy: I just look at the current prompt's timestamp and compare it to the last prompt's timestamp. But for any command which produces lots of "scroll off the end of the scroll-back buffer" output I lose the ability to figure out if a command ended shortly after I gave up waiting and went to bed, or whether it finished just before I got up in the morning.

Therefore I recently tweaked my bash command-line prompt, yet again, to give me the duration of the last command that was run.

There are two "tricks" that were necessary to get this working successfully. The first is the "SECONDS" environment variable bash provides. If you haven't undefined it, or otherwise used it for your own purposes, bash provides an environment variable called "SECONDS" which, when read, gives you the number of seconds since a given bash instance was started. The second trick is using bash's "trap" functionality with the "DEBUG" signal. When using trap: "if a sigspec is DEBUG, the command arg is executed before every simple command".

Keeping track of the duration of any command is a simple matter of storing the current SECONDS at the start of the command, then comparing that value to the current SECONDS at the end of the command. We know when any new command is about to start thanks to the DEBUG trap, and we know when the command is done thanks to bash printing a new PS1 prompt.